1. Field of the Invention
The present invention relates to an induction motor drive unit, a motor drive system, and an elevating system.
2. Description of the Related Art
An elevating system including an induction motor is known. Lifting goods with the elevating system such as a crane requires protection technology. For example, an excessive slip in the induction motor caused by an acceleration rate of the induction motor which is higher than a value determined by the weight of the goods should be prevented. Further, falling of the goods caused by a load variation due to an excessive load, an unstable state of the goods, and vibrations should be prevented. In addition, variations in constants of the induction motor due to heat should be prevented.
JP 02-262893 A discloses that a slip frequency is obtained from a difference between a rotating speed detected with a rotational speed sensor and an input frequency (primary frequency). When the slip frequency exceeds a predetermined value, deceleration or stopping is done.
JP 3351244 discloses at paragraphs 0033 and 0036 a technology of vector-control which controls currents in an induction motor to obtain a maximum torque at a low rotational speed range. In this technology, a d-axis current requiring no polarity setting is increased, and a q-axis current is limited to a predetermined value or approximately zero.